This invention relates to electronic photography for developing electrostatic images formed on the surface of electrostatic latent image bearing material with magnetic toner having relatively high resistivity, transferring the developed images and then fixing the transferred images. In particular, it relates to a process suitable for developing latent images on the surface of electrostatic latent image bearing material conveyed at a relatively high speed.
Such electronic photography comprises forming electrostatic latent images on the photoconductive surface of a photosensitive material; developing such electrostatic latent images to form toner images; fixing the toner images directly or transferring the toner images to a transferring member; and then fixing the images to produce a copy. In such electronic photography, there has been employed a two component developer which is mixed powder of non-magnetic toner and powdery iron carrier with development being carried out by a magnetic brush. The magnetic brush developing process with a two component developer involves electrifying the toner to a predetermined polarity by frictional electricity with the carrier and selectively sticking the toner on the surface of electrostatic latent image bearing material. The process is advantageous in that the transference is easily performed, etc. However, the process necessitates mixing means for electrifying the toner and carrier by the friction and replenishing means for replacing toner consumed during the development for keeping the toner concentration uniform so that the developing apparatus becomes extensive and complicated. In addition, as the carrier becomes fatigued after being used for a predetermined period of time, the carrier should be exchanged.
In order to overcome such disadvantages, there has been introduced one component magnetic toner comprising essentially a resin and magnetic powder as a developer. Known developing processes using such magnetic toner include the process disclosed in U.S. Pat. No. 3,909,258. This process comprises bearing electroconductive magnetic toner on an electroconductive shell and forming an electroconductive circuit between the surface of electrostatic latent image bearing material and electroconductive shell by connecting electrically the rear surface of the electrosatic latent image bearing material and electroconductive shell to perform the development by electrostatic induction. However, such a developing process is disadvantageous in that when the developed toner images are transferred onto a transferring member, the transferred images are distorted by the disappearance of charge in the toner, introduction of charge from the transferring electrode or the like due to the electroconductivity of toner. Hence the process is suitable for so-called PPC process wherein a copy is produced by developing, transferring and fixing images.
In order to accomodate use of magnetic toner for the PPC process, there has been proposed a developing process using insulating toner having enhanced electrical resistivity. For example, U.S. Pat. No. 4,131,931 discloses that in order to enhance the developing performance of insulating magnetic toner, toner is contacted electrically with an electrode to be electrified forcedly and the electrified toner is conveyed at a high speed to the latent image surface to increase the developing current, thereby improving the developing performance. However by such a process, the high conveying speed of toner necessitates the rotation of a non-magnetic cylindrical shell at a relatively higher speed so that the used toner is subjected to a strong force at the gap between the flow rate controller plate set within a relatively narrow size and the shell to agglomerate the magnetic toner or to stick it firmly on the surface of shell.
In addition, U.S. Pat. No. 4,102,305 discloses a process for improving the developing performance as well as the transference performance in the PPC process. This developing process comprises using special toner showing electroconductivity in the presence of a high electric field and insulation in a low electric field, applying high electric field onto the toner particle within the developing zone to impart electroconductivity to them to effect the development; and transferring the toner particles to the transferring zone to effect the transference under the condition free from high electric field. This patent illustrates an embodiment applying a voltage having a peak-peak value of 20-100 V/.mu.(20,000--1,000,000 V/cm) and frequency of 500-5000 Hz by connecting an AC source to the non-magnetic cylinder for forming an AC electric field between the photoconductive surface and the cylinder. However, when such a high AC voltage is applied, there is high possibility of forming pinholes through the electrostatic latent image bearing surface by the discharge. In addition, there may occur failure in the development of the predetermined electrostatic latent images due to introducing a charge from the magnetic toner turned to be electroconductive at the vicinity of peak value of AC electric field onto the electrostatic latent image bearing surface. In addition, the developing gap is significantly wide, e.g. approximately 1.65 mm in an example in said patent specification. Such a wide gap may scatter the toner or may form an immobile layer at the vicinity of the cylindrical surface of the magnetic brush if relatively poorly fluidal toner is used for the PPC process or may form agglomeration of toner by the formation of such an immobile layer thereby presenting a high possibility of failure in the development. Hence, such a developing gap is unsuitable for electronic photographing apparatus necessitating high reliability for a long period of time.